The term “thing in itself” means originally the object as it is, independent of the thinking subject’s cognition. For instance: A rainbow appears in the clouds; the rainbow is not a thing in itself, but the appearance of a thing in itself. The rainbow exists in man’s sensibility only. The colors of the spectrum, indeed all colors, the colors of the sky, of the clouds, of trees, of living beings, are sensations only; they are subjective phenomena, they are certain kinds of feelings representing objective realities, but they are not these objective realities themselves. They are perceived in the brain and are projected to a place outside the organism. The rainbow, as it is seen, is not a thing, but it is something seen, it is an appearance only. And this is true of all things seen and heard and perceived by any one of the senses. The sense-pictures are localised in space, they are projected outside to a spot where the combined experience of the senses has taught a sentient being to expect them. But all the objects of the objective world as they are perceived are and remain subjective sense-perceptions. The world of our senses around us is woven of our sensations. It is mere appearance. This is not a question concerning which there is any doubt, this is simply a matter of fact. But the question arises, “Can we know things as they are independent of sensation? Can we know things in themselves?” The physicist and every scientist is engaged with the problem, What are natural phenomena independent of sensation? Light is a sensation of vision, but what is the objective process that takes place when a human eye perceives light? The physicist answers this problem by eliminating in his mind the sense-element and by describing the facts of the process in terms of matter and motion. His answer is that light, objectively considered, is a certain vibration of the ether. If we can rely upon physical science, the thing in itself of a rainbow would be a certain refraction of ether-waves. These vibrations of the ether-waves are transmitted from the sun, and being broken in the falling raindrops actually take place independent of cognition, they are real whether we look at them or not.